An inhaler is developed which ejects micro-droplets of a medicine into an air flow path through which inhaled air via a mouthpiece flows by using an ejection principle in an ink jet system, and makes a user inhale (cf. Japanese Patent Application Laid-Open No. 2004-290593 and Japanese Patent Application Laid-Open No. 2004-283245). This type of an inhaler has an advantage of being capable of precisely spraying a specified amount of a medicine with a uniformized particle size.
A basic configuration of such a medicine ejection device includes an ejection head in which an ejection energy generating element such as an exothermic element is arranged, and a reservoir for storing the medicine to be supplied to the ejection head therein. When the reservoir is a simple closed container, as the medicine is ejected and the amount of the medicine in the reservoir decreases, a negative pressure is generated in the reservoir and the ejection performance decreases. For this reason, it has been necessary to take such countermeasures as the followings for the reservoir.
The countermeasure firstly includes a method of employing a structure in which a reservoir communicates with the atmosphere right before ejection starts. The structure is adopted in an ink jet printer which is a known art. However, when the reservoir stores a medicine in an amount of being inhaled multiple times, it is impossible to employ the structure of making the reservoir communicate with the atmosphere, because of needing to prevent a medicine remaining in the reservoir after the apparatus has been used once from changing its concentration and being denatured, so that the reservoir is required to have high gas barrier properties and sealability. The circumstance is the same also in the case when it is unfavorable for a medicine to contact air.
In the case, the reservoir may be a flexible container that has such a structure as to be crashed along with ejection (Japanese Patent Application Laid-Open No. 2004-290593). For instance, the reservoir can be made from a polyester film on which aluminum has been deposited. Alternatively, when a main body of the reservoir is a glass container or the like, it is necessary to make the reservoir into such a structure in which the one end of the container is blocked with a rubber plug, and the volumetric capacity of the reservoir decreases along with ejection.
In the case when the reservoir having such high sealability is used, a pressure difference between the outside and the inside of the reservoir increases as a medicine continues being ejected. Even when the negative pressure in the reservoir increases in this way, a considerable power is required to deform the flexible reservoir with a force caused by the negative pressure, or reduce the volumetric capacity by sliding the rubber plug with respect to the glass container. The above description will be made in the case of the glass container. When a force applied to the plug due to the negative pressure in the reservoir has exceeded the greatest static frictional force between the glass container and the plug, the plug starts moving. If the plug is more tightly adapted against the glass container in order to keep high sealability, the plug does not move until the negative pressure reaches a corresponding high value.
On the other hand, it is elucidated that as the negative pressure in the reservoir increases, a performance of the ejection from an ejection head decreases. When a medicine is ejected from a head with a nozzle diameter of 3 μm, an ejection amount does not decrease until the internal pressure of the reservoir reaches the vicinity of −5 kPa, but when having the internal pressure exceeds −5 kPa, the ejection amount decreases little by little, and when the internal pressure reached −20 kPa, the inhaler drew air through the ejection head and could not eject the medicine any more. For this reason, in order to realize a stable ejection, it is possible to keep the negative pressure in the reservoir to a predetermined value or lower (in the above described case, −5 kPa) as accurately as possible during the ejection period of time.
Because of this, an inhaler with the use of a conventional reservoir having high sealability decreases its ejection performance as the inhaler continues ejection, and sometimes could not eject a medicine though depending on a case.